Such a sorption sheet is known, e.g. from U.S. Pat. No. 4,298,796. The sorption sheet or support layer according to that patent comprises a filter mat of nitro-cellulose or glass fibre for supporting a plurality of beta particle emitting samples. This sheet is to be contacted with a scintillant to convert the beta particles into photons to be detected by means of a photodetector.
In use of this known sorption sheet radioactivity from one sample may diffuse towards an adjacent sample. Moreover, when the photodetector is monitoring a certain sample, photons from adjacent samples may strike the photodetector and cause an inaccuracy of measurement. This effect is normally called "optical cross-talk".